assert that ‘sort’ and ‘sortBy’ do not rely on Array#sort being stable

[davidchambers]

While working on #92 I realized I could replace idx: rs.length with idx: 0 and all the tests would still pass. This is because Array#sort is stable in my environment, as it is in most environments. The language specification does not guarantee this, though, so we should not assume it.

This pull request introduces a function, withUnstableArraySort, which monkey patches Array#sort, evaluates a thunk, then restores Array#sort to its prior value. Any change to Z.sort or Z.sortBy which assumes Array#sort to be stable will pass tests in the default environment but will fail some tests in the modified environment.

/cc @paldepind

[davidchambers]

This is not quite right. If an exception is thrown during the evaluation of thunk() we will fail to restore the original value.

[paldepind]

Sorry, I'm late 😭

This is because Array#sort is stable in my environment, as it is in most environments.

Array#sort is unstable in V8 from what I can find.

In any case, I think this PR is a very good idea to make the sort predictably unstable 👍